Limiting strain relaxation in iii-nitride hetero-structures by substrate and epitaxial layer patterning

ABSTRACT

A method of fabricating a substrate for a semipolar III-nitride device, comprising patterning and forming one or more mesas on a surface of a semipolar III-nitride substrate or epilayer, thereby forming a patterned surface of the semipolar III-nitride substrate or epilayer including each of the mesas with a dimension/along a direction of a threading dislocation glide, wherein the threading dislocation glide results from a III-nitride layer deposited heteroepitaxially and coherently on a non-patterned surface of the substrate or epilayer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. Section 119(e) ofco-pending and commonly-assigned U.S. Provisional Application Ser. No.61/406,876 filed on Oct. 26, 2010, by James S. Speck, Anurag Tyagi,Steven P. DenBaars, and Shuji Nakamura, entitled “LIMITING STRAINRELAXATION IN III-NITRIDE HETEROSTRUCTURES BY SUBSTRATE AND EPITAXIALLAYER PATTERNING,” attorney's docket number 30794.387-US-P1 (2010-804),which application is incorporated by reference herein.

This application is related to co-pending and commonly assigned U.S.Utility patent application Ser. No. ______, filed on same date herewith,by James S. Speck, Anurag Tyagi, Alexey Romanov, Shuji Nakamura, andSteven P. DenBaars, entitled “VICINAL SEMIPOLAR III-NITRIDE SUBSTRATESTO COMPENSATE TILT FO RELAXED HETERO-EPITAXIAL LAYERS,” attorney' docketnumber 30794.386-US-U1 (2010-973), which application claims the benefitunder 35 U.S.C. Section 119(e) of co-pending and commonly-assignedProvisional Patent Application Ser. No. 61/406,899, filed on Oct. 26,2010, by James S. Speck, Anurag Tyagi, Alexey Romanov, Shuji Nakamura,and Steven P. DenBaars, entitled “VICINAL SEMIPOLAR III-NITRIDESUBSTRATES TO COMPENSATE TILT FO RELAXED HETERO-EPITAXIAL LAYERS,”attorney' docket number 30794.386-US-P1 (2010-973), which application isincorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method to limit strain relaxation ofhetero-epitaxial III-nitride layers grown on III-nitridesubstrate/epilayers, by patterning said substrate/epilayer.

2. Description of the Related Art

(Note: This application references a number of different publications asindicated throughout the specification by one or more reference numberswithin brackets, e.g., [x]. A list of these different publicationsordered according to these reference numbers can be found below in thesection entitled “References.” Each of these publications isincorporated by reference herein.)

In spite of numerous advantages offered by growth of optoelectronicdevices on nonpolar/semipolar III-nitride substrates, misfit dislocation(MD) formation at misfitting heterointerfaces [1, 2] can make itdifficult for device manufacturers to fully realize the expectedinherent advantages. For semipolar III-nitride based devices, stressrelaxation via glide of pre-existing threading dislocations can limitthe composition/thickness of strained heteroepitaxial films that can begrown coherently on underlying substrates/films. This can, in turn,limit the device design space e.g. the range of emission wavelength forLight Emitting Diodes (LEDs)/Laser Diodes (LDs).

Additionally, performance for LDs can be affected due to poor opticalwaveguiding provided by thinner/lower composition waveguiding (typicallyInGaN) and cladding layers (typically AlGaN). The present inventionprovides a way to limit the stress-relaxation by the above mentionedglide process, and thus reduces the constraints on device design space,allowing employment of thicker/higher composition strained III-nitridealloy epitaxial layers. The proposed devices can be used as an opticalsource for various commercial, industrial, or scientific applications.These nonpolar or semipolar nitride LEDs and diode lasers can beexpected to find utility in the same applications as c-plane nitrideLEDs and diode lasers. These applications include solid-state projectiondisplays, high resolution printers, high density optical data storagesystems, next generation DVD players, high efficiency solid-statelighting, optical sensing applications, and medical applications.

SUMMARY OF THE INVENTION

The disclosed invention provides a method to limit strain relaxation ofhetero-epitaxial III-nitride layers grown on III-nitridesubstrate/epilayers, by patterning said substrate/epilayer. The presentinvention further includes growth and fabrication of devices onpatterned III-nitride substrates.

To overcome the limitations in the prior art described above, and toovercome other limitations that will become apparent upon reading andunderstanding the present specification, the present invention describesa semipolar or non-polar III-nitride device, comprising a semipolar ornonpolar III-nitride substrate or epilayer having a threadingdislocation density of 10⁶ cm⁻² or more; and a heterostructure,comprising semipolar or nonpolar III-nitride device layers, grown on thesubstrate or epilayer, wherein the heterostructure has a misfitdislocation density of 10⁴ cm⁻² or less.

The semipolar or nonpolar III-nitride substrate or epilayer can compriseone or more mesas with a dimension/along a direction of a threadingdislocation glide, thereby forming a patterned surface of the semipolaror nonpolar III-nitride substrate or epilayer, wherein theheterostructure is grown heteroepitaxially and coherently on thepatterned surface.

The dimension/can be between 10 micrometers and 1 millimeter.

At least one of the heterostructure's layers can have a differentIII-nitride composition from the semipolar or nonpolar III-nitridesubstrate or the epilayer.

A heterointerface between the heterostructure and the patterned surfacecan include a misfit dislocation density that is reduced by a factor ofat least 10, or at least 1000, as compared to a misfit dislocationdensity resulting from a semipolar or nonpolar III-nitrideheterostructure grown heteroepitaxially and coherently on anon-patterned surface of the semipolar or nonpolar III-nitride substrateor epilayer.

One or more of the semipolar or nonpolar III-nitride device layers canbe thicker, and have a higher alloy composition, as compared to (1)semi-polar or nonpolar III-nitride device layers that are grown on anon-patterned surface of a semi-polar or nonpolar III-nitride substrateor epilayer, or as compared to (2) semi-polar or nonpolar III-nitridedevice layers that are grown on a different patterned surface of thesemipolar or nonpolar III-nitride substrate.

The device can comprise a device structure for a non-polar or semi-polarIII-nitride Light Emitting Diode (LED) or Laser Diode (LD), wherein thedevice structure includes the heterostructure and one or more activelayers emitting light having a peak intensity at one or more wavelengthscorresponding to green wavelengths or longer, or a peak intensity at awavelength of 500 nm or longer.

The active layers can comprise III-nitride Indium containing layers thatare sufficiently thick, and have sufficiently high Indium composition,such that the LED or LD emits the light having the wavelengths.

The device structure can comprise waveguiding and/or cladding layers,comprising III-nitride layers that are sufficiently thick, and having acomposition, to function as the waveguiding and/or cladding layers forthe LD or LED.

The active layers and the waveguiding layers can comprise one or moreInGaN quantum wells with GaN barrier layers, and the cladding layers cancomprise one or more periods of alternating AlGaN and GaN layers.

One or more of the semi-polar or non-polar III-nitride device layers canhave a thickness greater than a (e.g., Matthews Blakeslee) criticalthickness for one or more semi-polar or non-polar III-nitride layersdeposited on a non-patterned surface of a semi-polar or non-polarIII-nitride substrate or epilayer.

The present invention further discloses a method of fabricating asubstrate for a semipolar or non-polar III-nitride device, comprisingpatterning and forming one or more mesas on a surface of a semipolar ornon-polar III-nitride substrate or epilayer, thereby forming a patternedsurface of the semipolar III-nitride substrate or epilayer, wherein eachof the mesas has a dimension/along a direction of a threadingdislocation glide, wherein the threading dislocation glide results froma III-nitride layer deposited heteroepitaxially and coherently on anon-patterned surface of a semi-polar or non-polar III-nitride substrateor epilayer.

A pre-existing threading dislocation density of the III-nitridesubstrate can be at least 10⁵ cm⁻² or between 10⁵ and 10⁷ cm⁻².

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout:

FIG. 1 is a perspective view schematic illustrating MD formation by TDglide for the exemplary case of a strained heteroepitaxial (Al,In)GaNlayer grown on a semipolar (11-22) GaN substrate, taken from [1].

FIG. 2 illustrates a top view (a) and side view (b) of mesas patternedon a substrate or epilayers, according to one or more embodiments of thepresent invention.

FIG. 3 is a flowchart illustrating a method of fabricating a device,according to one or more embodiments of the present invention.

FIG. 4 is a cross-sectional schematic of device heterostructure layerson a III-nitride substrate or epilayer, according to one or moreembodiments of the present invention.

FIG. 5 is a cross-sectional schematic of device heterostructure layerson a non-polar III-nitride substrate or epilayer, according to one ormore embodiments of the present invention.

FIG. 6 is a cross-sectional schematic of a device structure grown on thepatterned substrate, according to one or more embodiments of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

In the following description of the preferred embodiment, reference ismade to the accompanying drawings which form a part hereof, and in whichis shown by way of illustration a specific embodiment in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

Overview

State of the art commercial III-nitride devices are based on coherentgrowth of hetero-epitaxial films on a III-nitride substrate. Asmentioned above, this limits the thickness/composition of strainedIII-nitride films and limits the device design space. Using highercomposition strained epilayers leads to the formation of MDs atheterointerfaces, which can degrade device performance [1]. The currentinvention provides a work-around to the MD formation process by limitingthe glide length of pre-existing Threading Dislocations (TDs).

Nomenclature

GaN and its ternary and quaternary compounds incorporating aluminum andindium (AlGaN, InGaN, AlInGaN) are commonly referred to using the terms(Al,Ga,In)N, III-nitride, Group III-nitride, nitride,Al_((1-x-y))In_(y)Ga_(x)N where 0<x<1 and 0<y<1, or AlInGaN, as usedherein. All these terms are intended to be equivalent and broadlyconstrued to include respective nitrides of the single species, Al, Ga,and In, as well as binary, ternary and quaternary compositions of suchGroup III metal species. Accordingly, these terms comprehend thecompounds AlN, GaN, and InN, as well as the ternary compounds AlGaN,GaInN, and AlInN, and the quaternary compound AlGaInN, as speciesincluded in such nomenclature. When two or more of the (Ga,Al,In)component species are present, all possible compositions, includingstoichiometric proportions as well as “off-stoichiometric” proportions(with respect to the relative mole fractions present of each of the(Ga,Al,In) component species that are present in the composition), canbe employed within the broad scope of the invention. Accordingly, itwill be appreciated that the discussion of the invention hereinafter inprimary reference to GaN materials is applicable to the formation ofvarious other (Al,Ga,In)N material species. Further, (Al,Ga,In)Nmaterials within the scope of the invention may further include minorquantities of dopants and/or other impurity or inclusional materials.Boron (B) may also be included.

The term “Al_(x)Ga_(1-x)N-cladding-free” refers to the absence ofwaveguide cladding layers containing any mole fraction of Al, such asAl_(x)Ga_(1-x)N/GaN superlattices, bulk Al_(x)Ga_(1-x)N, or AlN. Otherlayers not used for optical guiding may contain some quantity of Al(e.g., less than 10% Al content). For example, an Al_(x)Ga_(1-x)Nelectron blocking layer may be present.

One approach to eliminating the spontaneous and piezoelectricpolarization effects in GaN or III-nitride based optoelectronic devicesis to grow the III-nitride devices on nonpolar planes of the crystal.Such planes contain equal numbers of Ga (or group III atoms) and N atomsand are charge-neutral. Furthermore, subsequent nonpolar layers areequivalent to one another so the bulk crystal will not be polarizedalong the growth direction. Two such families of symmetry-equivalentnonpolar planes in GaN are the {11-20} family, known collectively asa-planes, and the {1-100} family, known collectively as m-planes. Thus,nonpolar III-nitride is grown along a direction perpendicular to the(0001) c-axis of the III-nitride crystal.

Another approach to reducing polarization effects in (Ga,Al,In,B)Ndevices is to grow the devices on semi-polar planes of the crystal. Theterm “semi-polar plane” (also referred to as “semipolar plane”) can beused to refer to any plane that cannot be classified as c-plane,a-plane, or m-plane. In crystallographic terms, a semi-polar plane mayinclude any plane that has at least two nonzero h, i, or k Millerindices and a nonzero 1 Miller index.

Technical Description

For the case of semipolar III-nitride heteroepitaxy, significant stressrelaxation can be realized by the glide of pre-existing TDs. FIG. 1 is aperspective view schematic illustrating MD formation by TD glide for theexemplary case of a strained heteroepitaxial (Al,In)GaN layer 100 grownon a semipolar (11-22) GaN substrate 102. The MD line directioncorresponds to the intersection of the glide plane 104 (which is (0001)for basal plane slip) and the growth plane 106, which for FIG. 1corresponds to the in-plane m-axis [1-100]. Also shown is theheterointerface 108 between the (Al,In)GaN layer 100 and the GaNsubstrate 102, the non-patterned surface 110 of the substrate 102 uponwhich layer 100 is deposited, and the (11-22) and (1-1-23) directions.

As a simple estimate, the maximum MD density, ρ_(MD) ^(max), is given by

$\begin{matrix}{\rho_{MD}^{\max} \cong {\frac{1}{2}\rho_{TD}l}} & {{Equation}\mspace{14mu} (1)}\end{matrix}$

where l is the glide length of the TD and ρ_(TD) is the pre-existing TDdensity. In the absence of any crossing MDs, or other obstacles toglide, l should correspond to the wafer dimensions in the projectedglide direction, which for basal slip should correspond to theintersection of the (0001) c-plane 104 and the growth plane 106.Typically, the semipolar GaN substrates 102 are cross-cut from c-planeGaN boules, and the typical wafer dimensions are on the order of 1 cm×1cm. Thus, ρ_(MD) ^(max)˜10⁻⁶ cm⁻²×1 cm=10⁶ cm⁻¹, or the minimum MDspacing is on the order of 100 Angstroms (Å) which corresponds toplastic relaxation of the order of ˜2% (the relieved misfit strainaccompanying plastic relaxation is given by f_(misfit)=b_(edge,∥)·ρ_(MD)^(MAX), where b_(edge,∥) is the edge component of the MD Burgers vectorthat is parallel to the heterointerface 108).

The present invention notes that the MD density, and consequently stressrelaxation, has a direct dependence on the run length of the TDs. Thisenables the present invention to limit stress relaxation by limiting therun/glide length of the TDs. An expedient way to accomplish this is topattern “mesas” on the substrate/epilayers. This is illustrated in FIGS.2( a) and 2(b) (again for the exemplary case of (11-22)), wherein mesas200 a, 200 b, and 200 c are patterned into the surface of the (11-22)GaN substrate 202, to form a patterned surface 204 of the substrate 202.In FIG. 2( a)-(b), l₁, l₂, and l₃ are the mesa dimensions parallel tothe TD glide direction (m-axis [1-100], same as MD line direction) foreach of the 3 mesas 200 a, 200 b, and 200 c depicted in FIGS. 2( a) and2(b). Also shown are the height h (˜0.5 micrometers) of the mesas 200a-200 c, a surface area of the substrate 202 having a length L (˜1centimeter) along the direction of the TD glide direction (parallel (∥)to the [1-100] m-axis direction which is also shown), and the [-1-123]direction (parallel to the projection of the c-axis).

By using mesa dimensions of ˜10 μm to ˜1 mm in the TD glide direction,the TD glide length and MD density can be reduced by a factor of ˜10³ to˜10, and consequently, only very little misfit relief can be achieved byTD glide. This enables the present invention to grow strained epitaxiallayers with higher alloy composition and/or greater thickness withoutgenerating a high density of MDs at the misfitting heterointerfaces (MDdensity due to pre-existing TDs would be limited to

${\frac{1}{2}\rho_{TD}l},$

where/corresponds to the mesa dimension parallel to the glidedirection). As mentioned previously, this enables a wider and moreflexible device design space, which translates into improved performancefor devices.

FIG. 3 is a flow chart outlining the steps involved in one embodiment ofthe present invention.

The present invention starts with a semipolar III-nitride substrate orepilayer (grown on a substrate), as illustrated in Block 300. Thepre-existing TD density for the substrate/epilayer would typically be inthe 10⁵-10⁷ cm⁻² range.

Block 302 represents mesa patterning, wherein the substrate/epilayer isthen covered with a suitable mask (either photoresist or dielectric)having the desired mesa pattern, using conventional lithographictechniques. Although the mesa/mask pattern can take an arbitrary shape,the important dimension is the size of the pattern parallel to the glidedirection (l in equation 1) since that determines the maximum MD density(due to pre-existing TDs). The present invention is not limited topatterning using photoresist/dielectric masks (other methods may also beused).

Block 304 represents mesa etching, wherein the substrate/epilayer isthen etched to a suitable depth (>50 nm, e.g., 0.1-10 μm are typicaletch depths) to form mesa structures, and the mask is subsequentlyremoved (Block 306). The etching can include, but is not limited to, wetor dry etching.

The mesa sidewalls and the field may optionally be covered with adielectric, and the exposed substrate/epilayer surface may then besubjected to an ex situ or in situ cleaning or preparation, asrepresented in Block 308, prior to epitaxial re-growth of the deviceepitaxial structure (Block 310).

FIG. 4 illustrates a III-nitride heterostructure 400 on a substrate orepilayer 402, wherein ρ_(MD) ^(max)˜ρ_(TD)l and strain relaxation(∈^(relax)) in the heterostructure layers is ∈^(relax)=ρ_(MD)·b_(edge∥),and the c-projection direction and the semipolar a- or m-direction arealso shown.

FIG. 5 illustrates limiting strain relaxation in non-polar III-nitrideheterostructures (comprising heterostructure layers 500) by substratepatterning of a non-polar substrate 502 or epilayer. Also shown are MDs,the c-direction 504 of the III-nitride, the growth direction 506 of theheterostructure layers 500, and the m- or a-direction 508 of theIII-nitride. An l=1 cm wide wafer or substrate 502 with 10⁶ cm⁻² TDs,leads to 10⁶ MDs/cm² using the relation ρ_(MD) ^(max)˜ρ_(TD)l. If l isreduced to 100 micrometers or 10⁻² cm, the MD density can be reduced to10⁴ MDs/cm².

Device Embodiments

FIG. 1( a)-(b), FIG. 2( a)-(b), FIG. 5, and FIG. 6, illustrate variousdevice embodiments.

FIG. 5 illustrates a semipolar or nonpolar III-nitride substrate 502 orepilayer having a threading dislocation density of 10⁶ cm⁻² or more; andone or more semipolar or nonpolar III-nitride layers 500 a, 500 b, suchas device layers (or a heterostructure 500, comprising semipolar ornonpolar III-nitride layers or device layers 500 a, 500 b), grown (e.g.,coherently and/or heteroepitaxially) on the semipolar or nonpolarIII-nitride substrate 502 or epilayer, wherein the heterostructure 500or layers 500 a, 500 b have a misfit dislocation density of 10⁴ cm⁻² orless.

The substrate 502, 202 can be bulk III-nitride or a film of III-nitride.The substrate can comprise an initial semi-polar III-nitride (e.g.,template) layer or epilayer 502, 202 grown on a substrate (e.g.,heteroepitaxially on a foreign substrate, such as sapphire, spinel, orsilicon carbide).

FIG. 2( a), FIG. 2( b), and FIG. 5 illustrate the semipolar or nonpolarIII-nitride substrate 202, 502 or epilayer can comprise one or moremesas 200 a with a dimension/along a direction of a threadingdislocation glide, thereby forming a patterned surface 204 of thesemipolar or nonpolar III-nitride substrate or epilayer 502, 202 (thesurface 204 of the substrate comprises the mesas 200 a-c). Theheterostructure 500 or layers 500 a-b are grown heteroepitaxially and/orcoherently on the patterned surface 204.

The threading dislocation glide typically results from a nonpolar orsemipolar III-nitride layer 100 of the heterostructure depositedheteroepitaxially and coherently on a non-patterned surface 110 of anonpolar or semipolar III-nitride substrate 102 or epilayer (see FIG.1). The use of the patterned surface 204 can reduce or eliminate theamount of threading dislocation glide.

The dimension/can be, but is not limited to, between 10 micrometers and1 millimeter. The mesas 200 a can have a variety of shapes, including,but not limited to, square or rectangular shapes (as viewed from thetop).

At least one of the layers 500 a-b can have a different III-nitridecomposition from the nonpolar or semipolar III-nitride substrate 502 orthe epilayer.

A heterointerface 510 between the heterostructure 500, or layers 500a-b, and the patterned surface 204 can include a misfit dislocation (MD)density that is reduced by a factor of at least 10, or at least 1000, ascompared to a misfit dislocation density resulting from a semipolar ornonpolar III-nitride heterostructure 100 grown heteroepitaxially and/orcoherently on a non-patterned surface 110 of the nonpolar or semipolarIII-nitride substrate or epilayer 102.

FIG. 6 illustrates a device structure 600 comprising device layers thatcan be deposited on the patterned surface 204, wherein the devicestructure 600 or device layers are for a non-polar or semi-polarIII-nitride Light Emitting Diode (LED) or Laser Diode (LD) emittinglight. The device structure 600 includes the heterostructure 500, orlayers 500 a-b, or one or more active layers 602 emitting light having apeak intensity at one or more wavelengths corresponding to greenwavelengths or longer (e.g., yellow or red light), or a peak intensityat a wavelength of 500 nm or longer. The layers 500 a-b of theheterostructure 500 can be the active layers 602.

The present invention is not limited to devices emitting at particularwavelengths, and the devices can emit at other wavelengths. The devicecan be a blue, yellow, or red light emitting device, for example.

The active layer 602 can comprise one or more non-polar or semi-polarIII-nitride layers comprising Indium. The nonpolar or semipolarIII-nitride device layer 500 a-b or active layer 602 can be sufficientlythick, and have sufficiently high Indium composition, such that thelight emitting device emits the light having the desired wavelengths.The light emitting active layer(s) 602 can include InGaN layers, e.g.,one or more InGaN quantum wells with GaN barriers. The InGaN quantumwells can have an Indium composition of at least 7%, at least 10%, atleast 16%, or at least 30%, and a thickness greater than 4 nanometers(e.g., 5 nm), at least 5 nm, or at least 8 nm, for example. However, thequantum well thickness may also be less than 4 nm, although it istypically above 2 nm thickness.

The semipolar or nonpolar III-nitride device layers of the semi-polar ornon-polar light emitting device structure 600 can further include n-typewaveguiding layers 604 a and p-type waveguiding layers 604 b and/orn-type cladding layers 606 a and p-type cladding 606 b layers that aresufficiently thick, and have a composition, to function aswaveguiding/cladding layers for the light emitted by the active layers602 of the LD or LED. The waveguiding layers 604 a-b can comprise anindium composition of at least 7%, or at least 30%, for example.

The waveguiding layers 604 a, 604 b can comprise one or more InGaNquantum wells with GaN barrier layers, and the cladding layers 606 a,606 b can comprise one or more periods of alternating AlGaN and GaNlayers, for example. The device structure can be AlGaN cladding layerfree.

The device structure 600 can further comprise an AlGaN blocking layer608 and a GaN layer 610. While FIG. 6 illustrates a Laser Diodestructure, the structure can be modified as necessary to form a LightEmitting Diode structure.

One or more of the III-nitride semi-polar or non-polar device layers 500a-b can be heterostructures, or layers that are lattice mismatched with,and/or have a different composition from, another of the semi-polar ornon-polar III-nitride device layers or the substrate. For example, thedevice layers can be (Al,In)GaN layers on a GaN substrate. For example,the device layers can be InGaN layer(s) and an AlGaN layer(s), whereinthe heterointerface is between the InGaN layer and the AlGaN layer, theInGaN layer and a GaN layer, or an AlGaN layer and a GaN layer.

One or more of the semi-polar or semipolar III-nitride device layers 500a, 500 b can have a thickness equal to or greater than their criticalthickness on a non-patterned surface 110 of the nonpolar or semipolarIII-nitride substrate 102.

The equilibrium critical thickness corresponds to the case when it isenergetically favorable to form one misfit dislocation at thelayer/substrate interface.

Experimental, or kinetic critical thickness, is always somewhat orsignificantly larger than the equilibrium critical thickness. However,regardless of whether the critical thickness is the equilibrium orkinetic critical thickness, the critical thickness corresponds to thethickness where a layer transforms from fully coherent to partiallyrelaxed.

Another example of the critical thickness is the Matthews Blakesleecritical thickness [4].

For example, a total thickness 612 of all the active layers 600 (e.g.,multi-quantum-well stack thickness) can be equal to, or greater than,the critical thickness for the active layer on a non-patterned surface110. A total thickness 614 of the n-type or p-type waveguiding layers604 a, 604 b can be equal to, or greater than, the critical thicknessfor the waveguiding layers 604 a, 604 b on the non-patterned surface110. A total thickness 616 of the n-type or p-type cladding layers 606a, 606 b can be equal to, or greater than, the critical thickness forthe cladding layers 606 a, 606 b on the non-patterned surface 110.

However, using the patterned surface 204, the layers 602, 604 a, 604 b,and 606 b, 606 a can be coherently grown. For a layer X grown on a layerY, for the case of coherent growth, the in-plane lattice constant(s) ofX are constrained to be the same as the underlying layer Y. If X isfully relaxed, then the lattice constants of X assume their natural(i.e. in the absence of any strain) value. If X is neither coherent norfully relaxed with respect to Y, then it is considered to be partiallyrelaxed. In some cases, the substrate might have some residual strain.

Device structures using this method can be different because of thepossibility of a wider available device design space (e.g., defect-freecoherent structures with higher composition/thicker alloy layers).

One or more of the device layers 500 a-b can have a thickness and/orcomposition that is high enough such that a film, comprising all, or oneor more of, the device layers 500 a-b, has a thickness near or greaterthan the film's critical thickness for relaxation on a non-patternedsubstrate.

One or more of the semipolar or nonpolar III-nitride device layers 500a, 500 b can be thicker, and have a higher alloy composition (e.g., moreAl, In, and/or B, or non-gallium element), as compared to semi-polar ornonpolar III-nitride device layers that are grown on an a non-patterned,or different patterned surface, of a semi-polar or nonpolar III-nitridesubstrate or epilayer.

Accordingly, one or more embodiments of the present invention illustratea method to limit strain relaxation of hetero-epitaxial III-nitridelayers 500 grown on a III-nitride substrate or epilayer, comprisingpatterning the substrate or epilayer; and growing the III-nitride layers500 on the patterned substrate 202.

Embodiments of the present invention include growing, processing, and/orcontacting the device layers 500 a, 500 b on the patterned surface 204to fabricate any electronic or optoelectronic device, including, but notlimited to, an LED, a transistor, a solar cell, or a LD.

Possible Modifications

The substrate/epilayer can be grown using alternative techniques e.g.,Hydride Vapor Phase Epitaxy (HVPE)/Molecular Beam Epitaxy (MBE)/ChemicalVapor Deposition (CVD)/Metal Organic Vapor Deposition(MOCVD)/ammonothermal techniques etc. The process flow for patterningetched mesas can be different—e.g., positive/negative photoresist,various dielectric masks (SiO₂, silicon nitride, etc.) can be employedfor the etching (e.g., dry/wet etching).

Various etch chemistry and/or cleaning procedures can alternatively beemployed.

The device epitaxial re-growth can be performed using avariety/combination of growth techniques—e.g., HVPE, MBE, CVD, MOCVD, orammonothermal growth, etc. Additionally, if MDs are formed bypyramidal/prismatic slip, then the line direction of the MDs wouldchange accordingly; so the mesa dimensions would have to be modifiedaccordingly. In all cases, 1 (in Equation 1) corresponds to the TD runlength in the glide direction, and is the dimension of consequence.

ADVANTAGES AND IMPROVEMENTS

The present invention is applicable to electronic and optoelectronicdevices grown on III-nitride substrates (e.g., LEDs, LDs, solar cells,High Electron Mobility Transistors (HEMTs) etc.)

The invention provides a way to limit stress-relaxation in semipolarIII-nitride heteroepitaxy, thus providing an extended device designspace incorporating thicker/higher composition alloy epilayers. For LEDsor LDs, an expanded emission wavelength, e.g. green, yellow and red LEDsand LDs can be realized. Significantly improved optical waveguiding canbe achieved for LDs by using thicker/higher composition waveguiding andcladding layers.

State of the art semipolar III-nitride devices are grown on as receivedGaN substrates (typically grown by HVPE with TD density ˜10⁶ cm⁻²). Asdiscussed above, this implies there are enough pre-existing TDs torelieve a misfit stress of ˜2%. Alternatively, looking at Equation 1, MDdensity can also be limited by reducing TD density. In fact, GaNsubstrates grown by the ammonothermal method with TD density of 5×10⁴cm⁻² have been reported [3]. However, such substrates are not easilyavailable and the TD density strongly depends on the growth techniquesand growth conditions. HVPE grown GaN substrates which are commerciallyavailable have typical TD density of ˜10⁶ cm⁻². In contrast, the presentinvention can be applied to substrates/epilayers with varying degrees ofTD density and is thus not limited by pre-existing TD density. Thus, thepresent invention has fewer constraints and is widely applicable.

REFERENCES

The following references are incorporated by reference herein.

-   [1] Tyagi et al., Applied Physics Letters 95, 251905 (2009).-   [2] Young et al., Applied Physics Express 3, 011004 (2010).-   [3] Kucharski et al., Applied Physics Letters 95, 131119 (2009).-   [4] J. Matthews and A. Blakeslee, J. Cryst. Growth 32 265 (1976).

CONCLUSION

This concludes the description of the preferred embodiment of thepresent invention. The foregoing description of one or more embodimentsof the invention has been presented for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise form disclosed. Many modifications andvariations are possible in light of the above teaching. It is intendedthat the scope of the invention be limited not by this detaileddescription, but rather by the claims appended hereto.

1. A semipolar or non-polar III-nitride device, comprising: a semipolaror nonpolar III-nitride substrate or epilayer having a threadingdislocation density of 10⁶ cm⁻² or more; and a heterostructure,comprising semipolar or nonpolar III-nitride device layers, grown on thesemipolar or nonpolar III-nitride substrate or epilayer, wherein theheterostructure has a misfit dislocation density of 10⁴ cm⁻² or less. 2.The device of claim 1, wherein: the semipolar or nonpolar III-nitridesubstrate or epilayer comprises one or more mesas with a dimension/alonga direction of a threading dislocation glide, thereby forming apatterned surface of the semipolar or nonpolar III-nitride substrate orepilayer, and the heterostructure is grown heteroepitaxially andcoherently on the patterned surface.
 3. The device of claim 2, wherein lis between 10 micrometers and 1 millimeter.
 4. The device of claim 2,wherein at least one of the heterostructure's layers has a differentIII-nitride composition from the semipolar or nonpolar III-nitridesubstrate or the epilayer.
 5. The device of claim 2, wherein aheterointerface between the heterostructure and the patterned surfaceincludes a misfit dislocation density that is reduced by a factor of atleast 10 as compared to a misfit dislocation density resulting from asemipolar or nonpolar III-nitride heterostructure grownheteroepitaxially and coherently on a non-patterned surface of thesemipolar or nonpolar III-nitride substrate or epilayer.
 6. The deviceof claim 2, wherein a heterointerface between the heterostructure andthe patterned surface includes a misfit dislocation density that isreduced by a factor of at least 1000 as compared to a misfit dislocationdensity resulting from a semipolar or nonpolar III-nitrideheterostructure grown heteroepitaxially and coherently on anon-patterned surface of the semipolar or nonpolar III-nitride substrateor epilayer.
 7. The device of claim 1, wherein one or more of thesemipolar or nonpolar III-nitride device layers are thicker, and have ahigher alloy composition, as compared to: semi-polar or nonpolarIII-nitride device layers that are grown on a non-patterned surface ofsemi-polar or nonpolar III-nitride substrate or epilayer, or semi-polaror nonpolar III-nitride device layers that are grown on a differentpatterned surface of the semipolar or nonpolar III-nitride substrate. 8.The device of claim 1, further comprising a device structure on thepatterned surface, wherein: The device structure is for a non-polar orsemi-polar III-nitride Light Emitting Diode (LED) or Laser Diode (LD),and the device structure includes the heterostructure and one or moreactive layers emitting light having a peak intensity at one or morewavelengths corresponding to green wavelengths or longer, or a peakintensity at a wavelength of 500 nm or longer.
 9. The device of claim 8,wherein the active layers comprise III-nitride Indium containing layersthat are sufficiently thick, and have sufficiently high Indiumcomposition, such that the LED or LD emits the light having thewavelengths.
 10. The device of claim 9, wherein: the device structurecomprises waveguiding layers, comprising III-nitride layers that aresufficiently thick, and have a composition, to function as thewaveguiding layers for the LD or LED, or the device structure compriseswaveguiding and cladding layers, comprising III-nitride layers that aresufficiently thick, and have a composition, to function as waveguidingand cladding layers for the LD or LED.
 11. The device of claim 11,wherein the active layers and the waveguiding layers comprise one ormore InGaN quantum wells with GaN barrier layers, and the claddinglayers comprise one or more periods of alternating AlGaN and GaN layers.12. The device of claim 1, wherein: one or more of the semi-polar ornon-polar III-nitride device layers have a thickness and compositionthat is high enough such that a film, comprising the semi-polar ornon-polar III-nitride device layers, has a thickness near or greaterthan the film's critical thickness for relaxation, and the criticalthickness is for one or more semi-polar or non-polar III-nitride devicelayers deposited on a non-patterned surface of a semi-polar or non-polarIII-nitride substrate or epilayer.
 13. A method of fabricating asubstrate for a semipolar or non-polar III-nitride device, comprising:patterning and forming one or more mesas on a surface of a semipolar ornon-polar III-nitride substrate or epilayer, thereby forming a patternedsurface of the semipolar or non-polar III-nitride substrate or epilayer,wherein: each of the mesas has a dimension/along a direction of athreading dislocation glide, wherein the threading dislocation glideresults from a semi-polar or non-polar III-nitride layer depositedheteroepitaxially and coherently on a non-patterned surface of asemi-polar or non-polar III-nitride substrate or epilayer.
 14. Themethod of claim 13, wherein a pre-existing threading dislocation densityof the non-polar or semi-polar III-nitride substrate is at least 10⁵cm⁻², or between 10⁵ and 10⁷ cm⁻².
 15. The method of claim 13, wherein lis between 10 μm and 1 mm.
 16. The method of claim 13, furthercomprising growing a heterostructure, comprising semi-polar or non-polarIII-nitride device layers, coherently on the patterned surface, whereinat least one of the semi-polar or non-polar III-nitride layers has adifferent III-nitride composition from the nonpolar or semipolarIII-nitride substrate or epilayer.
 17. The method of claim 16, wherein aheterointerface between the heterostructure and the patterned surfaceincludes a misfit dislocation density that is reduced by a factor of atleast 10 as compared to a misfit dislocation density resulting from asemipolar or nonpolar III-nitride heterostructure grownheteroepitaxially and coherently on a non-patterned surface of thesemipolar or nonpolar III-nitride substrate or epilayer.
 18. The methodof claim 16, wherein a heterointerface between the heterostructure andthe patterned surface includes a misfit dislocation density that isreduced by a factor of at least 1000 as compared to a misfit dislocationdensity resulting from a semipolar or nonpolar III-nitrideheterostructure grown heteroepitaxially and coherently on anon-patterned surface of the semipolar or nonpolar III-nitride substrateor epilayer.
 19. The method of claim 16, wherein one or more of thesemipolar or nonpolar III-nitride device layers are thicker, and have ahigher alloy composition, as compared to: semi-polar or nonpolarIII-nitride device layers that are grown on an on-axis surface ofsemi-polar or nonpolar III-nitride substrate or epilayer, or semi-polaror nonpolar III-nitride device layers that are grown on a differentvicinal surface of the semipolar or nonpolar III-nitride substrate. 20.The method of claim 16, further comprising growing a device structure,comprising nonpolar or semipolar III-nitride layers for a non-polar orsemi-polar III-nitride Light Emitting Diode (LED) or Laser Diode (LD),on the patterned surface, wherein: the device structure includes theheterostructure and one or more active layers emitting light having apeak intensity at one or more wavelengths corresponding to greenwavelengths or longer, or a peak intensity at a wavelength of 500 nm orlonger.
 21. The method of claim 20, wherein the active layers compriseIII-nitride Indium containing layers that are sufficiently thick, andhave sufficiently high Indium composition, such that the LED or LD emitsthe light having the wavelengths.
 22. The method of claim 21, wherein:the device structure comprises waveguiding layers, comprisingIII-nitride layers that are sufficiently thick, and have a composition,to function as the waveguiding layers for the LD or LED, or the devicestructure comprises waveguiding and cladding layers, comprisingIII-nitride layers that are sufficiently thick, and have a composition,to function as waveguiding and cladding layers for the LD or LED. 23.The method of claim 22, wherein the active layers and the waveguidinglayers comprise one or more InGaN quantum wells with GaN barrier layers,and the cladding layers comprise one or more periods of alternatingAlGaN and GaN layers.
 24. The method of claim 13, wherein one or more ofthe semi-polar or non-polar III-nitride device layers have a thicknessand composition that is high enough such that a film, comprising thesemi-polar or non-polar III-nitride device layers, has a thickness nearor greater than the film's critical thickness for relaxation, and thecritical thickness is for one or more semi-polar or non-polarIII-nitride device layers deposited on a non-patterned surface of asemi-polar or non-polar III-nitride substrate or epilayer.
 25. Asubstrate for a semipolar III-nitride device, comprising: one or moremesas on a surface of a semipolar III-nitride substrate or epilayer,forming a patterned surface of the semipolar III-nitride substrate orepilayer, wherein: each of the mesas includes a dimension/along adirection of a threading dislocation glide.